1. Field of the Invention
The present invention generally relates to an exhaust emission purifying apparatus for removing nitrogen oxides (NOx) contained in an exhaust emission and, more particularly, to an exhaust emission purifying apparatus in which a selective catalytic reduction (SCR) system is employed.
2. Description of the Related Art
As an exhaust emission purifying apparatus for removing NOx contained in an exhaust emission, an SCR-type exhaust emission purifying apparatus as disclosed in Japanese Laid-open (Kokai) Patent Application Publication No. 2000-27627 has been proposed.
In such an exhaust emission purifying apparatus, a reduction catalyst (NOx purification catalyst) is provided in an exhaust system, and a reducing agent is added to an upstream side in the exhaust stream (it will be hereinafter abbreviated as “exhaust upstream side” throughout the description) with respect to the reduction catalyst by injection from a nozzle, to thereby create a catalytic reduction reaction between NOx in the exhaust emission and the reducing agent in order to purify the NOx to harmless components. Such a reduction reaction is performed with ammonia which has high reactivity with NOx. As an reducing agent, not only a urea aqueous solution which easily produces ammonia by being hydrolyzed with heat and water vapor in the exhaust emission, but various types of reducing agents including a precursor, such as ammonia aqueous solution or hydrocarbons (HC), can be used.
In such an SCR-type exhaust emission purifying apparatus as mentioned above, the reduction catalyst and the nozzle for injecting the reducing agent or the precursor thereof to the exhaust upstream side of the reduction catalyst are arranged in series. In addition to this in-series arrangement of the reduction catalyst and the nozzle, if there is needed to further dispose a filter for trapping particulate matters (PM) suspended in the exhaust emission, a lengthwise size of the exhaust emission purifying apparatus which occupies that of the exhaust system becomes larger making it difficult to suitably design layout of on-vehicle components. Namely, to achieve the SCR-type exhaust emission purifying apparatus equipped with a PM collection function as well as a NOx removal function, with such a forethought in which a coming exhaust emission control will be further tightened, it is necessary to contrive a novel layout where the PM collection filter is provided in addition to the nozzle and the reduction catalyst, so as to minimize the entire apparatus.